Although at present complex multi-coloured prints may be applied quite readily to planar surfaces by such printing techniques as offset, lithographic, flexographic, screen printing and the like, there is no satisfactory method for printing of contoured articles wherein a high degree of accuracy is required.
Three dimensional topographical maps for use by government mapping departments, forestry departments, mining departments and military forces and the like require precisely located markings of contour lines, roads, railway tracks, rivers, streams, vegetation areas and the like in up to five or even more different colours. While the markings on such three dimensional contour maps may be produced quite satisfactorily by hand such manual marking is far too slow and expensive for high volume production.
It has been proposed to produce large quantities of three dimensional topographical contour maps by screen printing the required indicia or markings on a planar sheet of thermoplastics material and then vacuum forming the printed sheet in a contoured mould. This process is not satisfactory in that the varying degrees of stretching that occur in the heated sheet as it is drawn into cavities of varying depth cause distortion in the print with resultant misalignment of the indicia or markings with their intended position on the surface of the contoured map. In an endeavour to alleviate this problem computer calculated distortions are incorporated into the planar print with a view to compensating for the distortions which might otherwise occur during the vacuum forming process.
Although the pre-distortion of the printed indicia does to some extent improve the finished product, printed contour maps of this kind are inherently inaccurate. Differing levels of resident stress in the thermoplastic sheets combined with minor differences in the properties of the batches of thermoplastics material cannot be calculated and thus compensated for. In addition slight differences in moulding conditions such as pre-heat temperature of the sheet plastics material, mould temperatures and the like effectively prevent consistent manufacture of precisely marked three dimensional contour maps by this process.